1. Field of the Invention
The present invention relates to a novel process for producing a porous polytetrafluoroethylene (hereinafter referred to as "PTFE") film. More particularly, the present invention relates to a process for producing a porous PTFE film having large reduced flow rates for various fluids and a narrow pore size distribution, in particular, a porous PTFE film having at least two layers which have different average pore sizes and are completely integrated together at their interface.
2. Description of the Prior Art
A porous PTFE film is widely used, for example, as a fine filtration filter medium for various gases and liquids in various fields, in particular, in the semiconductor industry, a concentration film for fruit juices in the food industry, and a separation and purification film in the fermentation industry.
Since PTFE has good chemical resistance, its porous film is used as a filtration filter medium of corrosive gasses and liquids and a separator in a cell for the separation of an electrolyte.
To provide a good filtration filter medium, the porous PTFE film should have a narrow pore size distribution and a large permeation amount per unit time for various fluids. It is known that, when a porosity or a pore size is constant, the permeation amount of the fluid depends on a thickness of the film. That is, as the film thickness decreases, the permeation amount increases. However, when the film thickness is made thin, the porous PTFE film tends to be deformed or the pore size tends to be changed by pressure during filtration. In an extreme case, the film is broken and does not work as the filtration filter. Further, a very thin porous film has very poor handleability, so that it may be damaged when it is process to assemble a filter module or it is set in a filter holder.
Hitherto, as processes for producing a porous PTFE film, following processes are known:
(1) A process comprising paste extruding or press molding an emulsion polymerized PTFE to obtain an unsintered body of PTFE and then stretching it to obtain a porous material (Japanese Patent Publication No. 18991/1976) or a process comprising stretching a semi-sintered PTFE material (Japanese Patent Kokai Publication No. 152825/1984).
To produce a thin porous film of PTFE, a draw ratio is made large when a thickness of the unstretched film is the same, or a thickness of the unstretched film is made as thin as possible when the draw ratio is the same.
Since the average pore size will increases as the draw ratio increases, to maintain a pore size suitable for the functions of the porous film, the extremely large draw ratio cannot be employed. On the other hand, the thickness of the unstretched film which is prepared by paste extrusion or press molding has a lower limitation, though it has been tried to decrease the thickness.
(2) A process comprising stretching a semi-sintered PTFE material consisting of PTFE particles to obtain a porous PTFE article (Japanese Patent Kokai Publication No. 61827/1986). However, by this process, it is difficult to produce a thin film product since the PTFE particles are used.
(3) A process comprising forming a homogeneous mixture of a PTFE resin dispersion and a fiber-forming polymer and removing the fiber-forming polymer from a formed product to obtain a porous film (Japanese Patent Kokai Publication No. 34407/1989). However, the porous film produced by this process is not suitable for the semiconductor industry and the food industry since impurities such as inorganic and organic materials tend to contaminate the semiconductor or the food. In addition, steps of this process are complicated.
To provide a porous PTFE film having a thin thickness and a large transmission amount per unit time, some multi-layer porous films of PTFE comprising a filtering layer having a small pore size and a supporting layer having a larger pore size than that of the filtering layer have been proposed.
As processes for preparing such multi-layer films, following processes have been proposed:
(i) A process comprising laminating and contact bonding at least one porous PTFE film having a small pore size both in unsintered states and at least one porous PTFE film having a large pore size and heating and sintering the laminate product at a temperature higher than a melting point of PTFE to obtain a multi-layer porous PTFE film (Japanese Patent Kokai Publication No. 97686/1979).
(ii) A process comprising stretching an unsintered PTFE film between a low rotational speed roll and a high rotational speed roll while establishing a temperature difference and a compressive force in a direction of the film thickness to produce a porous film having different pore sizes on two surfaces of the film (Japanese Patent Publication No. 48562/1988).
Although it is for the production of a separation and concentration film of an isotope mixture gas but not for the production of the fine filtration filter, the following process is proposed for the production of a microporous separation film:
(iii) A process comprising laminating at least one PTFE thin film containing a pore-forming liquid agent and at least one other PTFE thin film containing a pore-forming liquid agent, pressing them to adhere and extracting the pore-forming liquid agents to form pores, whereby a multi-layer PTFE film comprising at least two layers which have different average pore sizes is produced (Japanese Patent Publication No. 22504/1980).
The above process (i) requires steps for separately producing at least two sheets or films having different porosities and for sintering the sheets or films while laminating and contact pressing them. To laminate very thin films or films having very small strength in the industrial scale production, very expensive equipment and very high skill are necessary in view of formation of wrinkles or breakage of the films.
The process (ii) stretches the films between the rolls in only one direction and cannot be used for biaxial stretching.
In the process (iii), the multi-layer film having different average pore sizes in the different layers is produced using a difference of packing densities of the emulsion polymerized PTFE powders which have different primary particle sizes and shapes, and a difference of types of the pore-forming agents to be used. However, the formed pores consist of spaces between the PTFE particles. This is explained in detail. An unsintered product produced by paste extrusion of the emulsion polymerized PTFE polymer particles has a structure like a closest packing material of primary particles. A specific gravity of the primary particles is from 2.1 to 2.3, while a specific gravity of the product as a whole is from 1.5 to 1.6 when the product is produced using a petroleum base solvent. The difference between the specific gravities is due to the spaces, and the spaces between the particles form pores. Anyway, the film having such structure has a very poor fluid permeating property as a filter, and its strength is very low in comparison to a sintered material. But, when the multi-layer porous film is sintered to increase the strength, the film becomes poreless so that it cannot be used as a fluid filter.
Japanese Patent Kokai Publication No. 131236/1982 proposes a process for producing a multi-layer porous film comprising laminating rolled PTFE sheets containing an additive, further rolling the laminate and stretching it. Though the porous film produced by this process has high strength, there is no difference of the pore size between the layers.
An asymmetric film comprising a very thin filter layer and a supporting layer having a pore size larger than that of the filter layer can be produced from cellulose acetate or polysulfone. Since this asymmetric film is produced by a wet coagulation method, a material of the film should be soluble in a solvent. Therefore, this process cannot be applied to PTFE which is insoluble in any solvent.